How to have a nice visual join between two colliding meshes?
In 3D modeling this is often quite difficult.

You could make them the same mesh and join the geometry, then use a vertex-group or vertex-colours to blend between 2 materials (most obvious solution, check on tutorials for blending between materials).

You could cheat...

add grass (particle systems), pebbles, rocks etc - this is what happens in real life isn't it? It will likely increase your render times but hide the join.
you could apply alpha to the lower part of the mesh that meets the ground so you don't see a sharp change.
You could add a strip of faces which sits just above the join and blends between the 2 surfaces (with alpha on both sides so you don't see any hard edges), I don't recommend this but it can be made to work, you just have to be careful that lighting is ok and its not casting shadows on the faces below it.
You could render them on different layers and composite them, alpha blending parts of the lower mesh. (I don't recommend this but adding for completeness)